The present invention generally relates to protective coatings, and more particularly to erosion- and impact-resistant coatings suitable for use in gas turbine engines.
Gas turbines, including gas turbine engines, generally comprise a compressor, a combustor within which a mixture of fuel and air from the compressor is burned to generate combustion gases, and a turbine driven to rotate by the combustion gases leaving the combustor. Both the compressor and turbine utilize blades with airfoils against which air (compressor) or combustion gases (turbine) are directed during operation of the gas turbine engine, and whose surfaces are therefore subjected to impact and erosion damage from particles entrained in the air ingested by the engine. Turboshaft engines used in helicopters are particularly prone to ingesting significant amounts of particulates when operated under certain conditions, such as in desert environments where sand ingestion is likely.
Though both are attributable to ingested particles, impact damage can be distinguished from erosion damage. Impact damage is primarily caused by high kinetic energy particle impacts, and typically occurs on the leading edge of an airfoil. Traveling at relatively high velocities, particles strike the leading edge or section of the airfoil at a shallow angle to the pressure (concave) surface of the airfoil, such that impact with the leading edge is head-on or nearly so. Because the airfoil is typically formed of a metal alloy that is at least somewhat ductile, particle impacts can deform the leading edge, forming burrs that can disturb and constrain airflow, degrade compressor efficiency, and reduce the fuel efficiency of the engine. Erosion damage is primarily caused by glancing or oblique particle impacts on the pressure side of an airfoil, and tends to be concentrated in an area forward of the trailing edge, and secondarily in an area aft or beyond the leading edge. Such glancing impacts tend to remove material from the pressure surface, especially near the trailing edge. The result is that the airfoil gradually thins and loses its effective surface area due to chord length loss, resulting in a decrease in compressor performance of the engine. Compressor blades suffer from both impact and erosion damage, but are particularly susceptible to impact damage along their leading edges, as well as erosion damage on their pressure (concave) surfaces.
Compressors of gas turbine engines of the type used in helicopters are often fabricated as blisks, in which a disk and its blades are manufactured as a single integral part, as opposed to manufacturing the disk and blades separately and then assembling the blades on the disk. The blades of a blisk are typically protected with a coating that may be deposited using various techniques, including physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes. The effectiveness of a protective coating on the blades of a blisk is particularly important since the entire blisk must be removed from the engine if sufficient erosion or impact damage has occurred. Coating materials widely used to protect blisk blades are generally hard, erosion-resistant materials such as nitrides and carbides. For example, see U.S. Pat. No. 4,904,528 to Gupta et al. (titanium nitride coatings), U.S. Pat. No. 4,839,245 to Sue et al. (zirconium nitride coatings), and U.S. Pat. No. 4,741,975 to Naik et al. (tungsten carbide and tungsten carbide/tungsten coatings). While exhibiting suitable erosion resistance, hard coating materials such as titanium nitride are not as resistant to impact damage. Greater impact resistance has been achieved with relatively thick coatings formed of tungsten carbide and chromium carbide applied by a high velocity oxy-fuel (HVOF) deposition process to thicknesses of about 0.003 inch (about 75 micrometers). As known in the art, HVOF deposition is a thermal spray process by which particles entrained in a supersonic stream of hydrogen and oxygen undergoing combustion are directed at a surface, and the softened particles deposit as “splats” to produce a coating having noncolumnar, irregular flattened grains and a degree of inhomogeneity and porosity.
The required thickness of these coating materials can result in excessively heavy coatings that may negatively affect the blade fatigue life (for example, high-cycle fatigue (HCF)), and for that reason the coatings are often applied to only the pressure side of a blade near the blade tip. Furthermore, while HVOF-deposited tungsten carbide and chromium carbide coatings perform well when subjected to relatively round particles found in desert sands, these coatings tend to exhibit higher rates of erosion when subjected to more aggressive particles, such as crushed alumina and crushed quartz, whose shapes tend to be more irregular with sharp corners.
PVD processes such as sputtering or electron beam physical vapor deposition (EB-PVD) deposit coatings that are microstructurally different from HVOF coatings in terms of being denser and/or having columnar microstructures. When deposited by these PVD processes, hard erosion-resistant materials such as nitrides and carbides perform better in terms of erosion resistance when subjected to aggressive media such as crushed alumina and crushed quartz. However, PVD-deposited coatings, which also differ from HVOF-deposited coatings in terms of mechanical properties such as ductility and elastic modulus, are susceptible to cracking and delamination when bombarded with round particles.
In view of the above, there is a need for coating materials that exhibit both erosion resistance and impact resistance for use as protective coatings on gas turbine blades, and particularly compressor blades of helicopters and other aircraft that operate in desert environments. It would also be desirable if such coatings were effective without contributing excessive weight to the compressor or adversely affecting desirable properties of the blades, such as fatigue life.